Mono

Mono provides the
necessary software to develop and run .NET client and server
applications.

Mono includes compilers, an ECMA-compatible runtime engine (the Common
Language Runtime, or CLR), and many libraries.

The libraries include Microsoft .NET compatibility libraries
(including ADO.NET, System.Windows.Forms and ASP.NET), Mono's own and
third party class libraries.Gtk#, a set of .NET bindings for the gtk+
toolkit and assorted GNOME libraries can be found in the
latter.

This library allows you to build fully native Gnome
application using Mono and includes support for user interfaces built
with the Glade interface builder.

Furthermore, Mono's runtime can be embedded into applications
for simplified packaging and shipping.